We hear this a lot. Everyone tells you that's what you do to make gold. So how do you know when something is low and something is high? It is much easier if its something you buy for yourself. If I run to the AH to get a belt buckel and the price looks a little high, I do the math on the mats. I did this one time and the price was way over priced. More than double the price of the mats. While I did need one now, it showed me that it is something that I should be selling.
I picked up a few stacks of the mats, found someone to craft them, and resold them in a few days while the market was up. Some things like this are just a good market. If I was a blacksmith, I would have stayed in it.
The other day I needed a epic gem. When I checked the AH, none were listed raw. So I posted a few I had at a price that averaged the cut gems. A good 50-60G markup over what I prabably picked the gem up for.
If you were paying attention a moment ago, I said I needed a epic gem but I had some to post. I buy stuff I will need later when I see its price low ahead of time. I knew I would prabably use the gem I had, but still felt I should check the AH for a cheaper one.
If I don't have the item I need, I will do the math on the raw mats and use them if it will save me gold. I know if I take everything with me to our raid, someone there can make whatever I need that I cannot make. If I am expecting to get a piece of gear, I will try to take those mats with me. When I got my T9.25 helm, it was in the middle of the raid. I had all the mats on me to gem it.
Even though I have more gold than I could ever spend, I still have a hard time overpaying for stuff. If I have to have it I will spend the gold, but saving 20G from buying smart is as good as a 20G sale.